Hi Samran14,
To my knowledge, cease and desist letters are written to prevent illegal activity from continuing, and are often made by courts. These letters are created to warn of potential litigation if the recipient continues their behaviour.
In your case defamatory statements aren’t allowed so yes the letter can be written.
I would suggest asking your lawyers for information regarding formatting and whom it is to be written by or if the matter must be taken to court for an order of cease and desist to be made.
One question I have is what are the facts? And what do you mean by they contacted you by demand letter?